fix: navigation bar color not updating on per-page basis

Root cause: _setNavBarColor used _lastNavColor (a module-level cache)
to skip wx.setNavigationBarColor when the color appeared unchanged.
But wx.setNavigationBarColor is per-page — each tab page has its own
navigation bar and needs an explicit call. The cache prevented
subsequent pages from ever receiving the call.

Example trace:
  index onShow → _setNavBarColor(#FF6B35) → _lastNavColor=#FF6B35 ✓
  settings onShow → _setNavBarColor(#FF6B35) → cache hit, return ✗
  User picks green → setTheme → _setNavBarColor(#43A047) ✓
  index onShow → _setNavBarColor(#43A047) → cache hit, return ✗
  → Index still shows orange!

Fix: remove _lastNavColor guard entirely. Keep setTimeout debounce.

utils/theme.js

@@ -44,7 +44,6 @@ const THEMES = [
 const BASE_VARS = '--text:#333;--text-secondary:#999;--bg:#F5F5F5;--card-bg:#FFFFFF;--border:#EEE;--success:#4CAF50;--primary:#FF6B35;--primary-light:#FF8C5A;--primary-bg:#FFF3ED;--primary-rgb:255,107,53;'
 const DEFAULT_THEME_ID = 'orange'
 const THEME_KEY = 'app_theme'
-let _lastNavColor = ''
 let _navColorTimer = null
 
 const getThemeById = (id) => THEMES.find(t => t.id === id) || THEMES[0]
@@ -62,8 +61,6 @@ const getCurrentTheme = () => {
 }
 
 const _setNavBarColor = (primary) => {
-  if (_lastNavColor === primary) return
-  _lastNavColor = primary
   if (_navColorTimer) clearTimeout(_navColorTimer)
   _navColorTimer = setTimeout(() => {
     try {